Optimised PCBM electron transport layer in inverted lead-free Cs3Bi2I9 flexible perovskite solar cells via FIRA

Flexible perovskite solar cells (FPSCs) offer significant versatility for portable and wearable technologies owing to their light weight, easy fabrication, low cost, and bendable properties. However, the commercialization of FPSCs faces challenges, particularly in terms of electron extraction efficiency and charge recombination, which impact device stability. Traditional high-temperature annealing methods are impractical for FPSCs due to their high energy consumption and environmental concerns. This study introduces a novel approach using flash infrared annealing (FIRA) to optimize a [6,6]-phenyl C61 butyric acid methyl ester (PCBM) electron transport layer (ETL) for lead-free cesium bismuth iodide (Cs₃Bi₂I₉) FPSC fabrication. The optimal FIRA conditions, 500 watts of power, PCBM concentration of 0.135 mol/L, and a 2-second annealing time were determined to enhance electron extraction, reduce charge recombination, and improve the overall device efficiency. Characterisation techniques, including UV-vis spectroscopy, photoluminescence, X-ray diffraction (XRD), and scanning electron microscopy (SEM), confirmed these optimisations. The optimised device achieved a power conversion efficiency (PCE) of 1.08%. By optimising the PCBM ETL FIRA, the PCE of lead-free Cs₃Bi₂I₉ FPSC was enhanced from 0.10% to 1.08%, representing a good improvement, along with a significant enhancement in electron extraction. These findings highlight the potential of optimised PCBM layers to improve the performance of FPSCs and contribute to their commercial viability.